Messages in this thread | | | From | Ilias Apalodimas <> | Date | Tue, 24 Oct 2023 11:26:56 +0300 | Subject | Re: [PATCH net-next v12 1/5] page_pool: unify frag_count handling in page_pool_is_last_frag() |
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On Mon, 23 Oct 2023 at 15:27, Yunsheng Lin <linyunsheng@huawei.com> wrote: > > On 2023/10/23 19:43, Ilias Apalodimas wrote: > > Hi Yunsheng, > > > > [...] > > > >> + * 1. 'n == 1': no need to actually overwrite it. > >> + * 2. 'n != 1': overwrite it with one, which is the rare case > >> + * for pp_frag_count draining. > >> * > >> - * The main advantage to doing this is that an atomic_read is > >> - * generally a much cheaper operation than an atomic update, > >> - * especially when dealing with a page that may be partitioned > >> - * into only 2 or 3 pieces. > >> + * The main advantage to doing this is that not only we avoid a atomic > >> + * update, as an atomic_read is generally a much cheaper operation than > >> + * an atomic update, especially when dealing with a page that may be > >> + * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count > >> + * handling by ensuring all pages have partitioned into only 1 piece > >> + * initially, and only overwrite it when the page is partitioned into > >> + * more than one piece. > >> */ > >> - if (atomic_long_read(&page->pp_frag_count) == nr) > >> + if (atomic_long_read(&page->pp_frag_count) == nr) { > >> + /* As we have ensured nr is always one for constant case using > >> + * the BUILD_BUG_ON(), only need to handle the non-constant case > >> + * here for pp_frag_count draining, which is a rare case. > >> + */ > >> + BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1); > >> + if (!__builtin_constant_p(nr)) > >> + atomic_long_set(&page->pp_frag_count, 1); > > > > Aren't we changing the behaviour of the current code here? IIRC is > > atomic_long_read(&page->pp_frag_count) == nr we never updated the atomic > > pp_frag_count and the reasoning was that the next caller can set it > > properly. > > If the next caller is calling the page_pool_alloc_frag(), then yes, > because page_pool_fragment_page() will be used to reset the > page->pp_frag_count, so it does not really matter what is the value > of page->pp_frag_count when we are recycling a page. > > If the next caller is calling page_pool_alloc_pages() directly without > fragmenting a page, the above code is used to ensure that pp_frag_count > is always one when page_pool_alloc_pages() fetches a page from pool->alloc > or pool->ring, because page_pool_fragment_page() is not used to reset the > page->pp_frag_count for page_pool_alloc_pages() and we have removed the > per page_pool PP_FLAG_PAGE_FRAG in page_pool_is_last_frag(). > > As we don't know if the caller is page_pool_alloc_frag() or > page_pool_alloc_pages(), so the above code ensure the page in pool->alloc > or pool->ring always have the pp_frag_count being one.
Fair enough, Jakub pulled the series before I managed to ack them, but that's okay. It's been long overdue apologies. FWIW I went through the patches and didn't find anything wrong coding-wise
Thanks /Ilias > > > > > > >> + > >> return 0; > >> + } > >> > >> ret = atomic_long_sub_return(nr, &page->pp_frag_count); > >> WARN_ON(ret < 0); > >> + > >> + /* We are the last user here too, reset pp_frag_count back to 1 to > >> + * ensure all pages have been partitioned into 1 piece initially, > >> + * this should be the rare case when the last two fragment users call > >> + * page_pool_defrag_page() currently. > >> + */ > >> + if (unlikely(!ret)) > >> + atomic_long_set(&page->pp_frag_count, 1); > >> + > >> return ret; > >> } > >> > > > > [....] > > > > Thanks > > /Ilias > > > > . > >
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